When oligomerizing light olefins within a refinery, there is frequently a desire to have the flexibility to make high octane gasoline, high cetane diesel, or combination of both. However, catalysts that make high octane gasoline typically make product that is highly branched and within the gasoline boiling point range. This product is very undesirable for diesel. In addition, catalysts that make high cetane diesel typically make product that is more linear and in the distillate boiling point range. This results in less and poorer quality gasoline due to the more linear nature of the product which has a lower octane value.
The oligomerization of butenes is often associated with a desire to make a high yield of high quality gasoline product. There is typically a limit as to what can be achieved when oligomerizing butenes. When oligomerizing butenes, dimerization is desired to obtain gasoline range material. However, trimerization and higher oligomerization can occur which can produce material heavier than gasoline such as diesel. Efforts to produce diesel by oligomerization have failed to provide high yields except through multiple passes.
When oligomerizing olefins from a fluid catalytic cracking (FCC) unit, there is often a desire to maintain a liquid phase within the oligomerization reactors. A liquid phase helps with catalyst stability by acting as a solvent to wash the catalyst of heavier species produced. In addition, the liquid phase provides a higher concentration of olefins to the catalyst surface to achieve a higher catalyst activity. This liquid phase in the reactor may be maintained by recycling heavy oligomerate to the oligomerization reactor.
To maximize propylene produced by the FCC unit, refiners may contemplate oligomerizing FCC olefins to make heavy oligomerate and recycling heavy oligomers to the FCC unit for cracking down to propylene.
Recycling heavy oligomers to the oligomerization unit or the FCC unit requires the heavy oligomers to be separated from lighter hydrocarbons in the oligomerate effluent typically in a fractionation column. A recycle ratio of oligomerate to feed can exceed 1. Transporting and separating heavy oligomerate recycled to the fractionation column exacts high utility costs.
Efficient processes and apparatuses are desired to recycle heavy oligomerate.